1. Field of the Invention
This invention relates to the field of bulk material handling. More precisely, it relates to devices employing nuclear radiation to measure amounts of specific constituents in bulk materials in a continuous process.
2. Description of the Prior Art
A Prompt Gamma Neutron Activation Analyzer, or PGNAA device bombards bulk amounts of material, such as limestone, coal, sand, mineral ores, wheat and the like with neutrons causing specific constituents in these materials to respond by issuing gamma rays that are subsequently measured to indicate concentrations of these constituents. Constituents such as calcium oxide (CaO) in limestone, sulfur (S) in coal, moisture (H2O) in sand, iron (Fe) in mineral ores, and proteins containing nitrogen (N) in wheat, are determined by using such a PGNAA. The bulk materials are delivered to the PGNAA on a rubber conveyor belt and passed through a radiation chamber where the materials are exposed to the neutron radiation. The gamma radiation caused thereby is registered on instruments that provide a direct readout of these constituents. Such a real-time analysis is needed in order to insure accurate tracking of concentrations of these constituents to provide a basis for delivering bulk materials carrying an accurate amount of whatever constituent is desired.
Depending on the type of process or application, the flow rate or mass loading of material to be measured in the PGNAA is not constant, resulting in variable material loads per unit length inside the radiation chamber. Variations of material loading can be continuous and can extend from empty to full PGNAA capacity.
The typical practice in the prior art of setting up and calibrating a PGNAA is to load the conveyor belt with a sample of the bulk material, in the particle size distribution and average tonnage rate (translated into the mass loading per unit length of the bed of material on the conveyor belt) expected in operation, where the amount of the constituent to be measured is accurately known. This xe2x80x9cstandardxe2x80x9d bulk material is usually mixed extremely well and many samples of the mix are taken and evaluated by chemical and other means to determine the exact concentration of the specific constituent. A large quantity of this mix, in sufficient size to emulate a real-time pass-through of the bulk material, is then tested under the PGNAA and the measurement instruments adjusted to indicate the amount of constituent that is already known in the xe2x80x9cstandardxe2x80x9d material.
This xe2x80x9cstandardxe2x80x9d material is expensive to make, difficult to keep isolated, costly to store, and the numerous tests run on it are expensive and time-consuming. In addition, constant tonnage flow rate through the PGNAA (constant mass loading per unit length) is difficult if not impossible, to achieve and maintain and surges in product create changes in tonnage and flow rates. It has been shown that these departures from desired optimum flow rate causes deterioration in the accuracy of the measurements. This leads to sales or quality of bulk material too rich or too lean in one or major specific constituents.
If the PGNAA is calibrated with a xe2x80x9cstandardxe2x80x9d of a given material mass loading, it will measure elemental composition accurately only when analyzing that same material mass loading. The PGNAA will produce significant measurement errors when analyzing materials of a different mass from that used in calibration. In general, PGNAA devices produce larger measurement errors when the mass loading is lower, and smaller errors when the mass loading is higher than the mass loading contained in the xe2x80x9cstandardxe2x80x9d during the calibration. The technical reasons for the PNGAA measurement errors at non-calibrated tonnage or mass loadings are described below. The consequence of this phenomenon is that analysis measurements of variable material streams are not accurate and not reliable enough for process control.
The amount of material flow is measured by a conventional weigh scale or flow meter and reported continuously and instantaneous as F in units of tons per hour (TPH). Given the conveyor belt speed B in units of meters per second (m/s), the instantaneous material loading L in mass units of kilograms per meter (kg/m) can be determined by:
L=F/(3.6xc3x97B)xe2x80x83xe2x80x83Equation I. 
Using Equation 1, if the belt speed B=1.95 m/s and the F=400, 800, and 1200 TPH, then loadings L=56.98, 113.96 and 170.94 kg/m respectively. Conversely, the tonnage flow rate through the PGNAA can be calculated by:
F=3.6xc3x97Bxc3x97Lxe2x80x83xe2x80x83Equation 2. 
The technical reasons for the PNGAA measurement errors at non-calibrated tonnage or mass loadings are caused by non-constant amounts of constituent signal emanating: (1) from: the conveyor belt, and (2) from the walls, irradiating, shielding, detectors and construction materials used inside the PGNAA device itself. Constituent signals emanating from any source other than the bulk material to be measured are referred to as xe2x80x9cbackground signalxe2x80x9d.
Conveyor belts used in the coal, cement, and mineral ore industries are one source of constituent PGNAA background. These belts are primarily Styrene Butadiene Rubber (SBR), in approximately a 1:4 blend of Styrene and Butadiene respectively. Styrene is C8H8 and Butadiene is C4H6. Additives to the SBR rubber include xcx9c0.5% sulfur for vulcanization, nylon or polyester cords for reinforcement, 10-30% oil and 10-15% CaCO3 for flexibility, and a few percent SiO2 and Al2O3 for improved wear resistance.
The materials of the conveyor belt and the walls and internals of PGNAA itself, referred to as xe2x80x9cbackground materialsxe2x80x9d, will capture neutrons and emit gamma rays and produce PGNAA signal just as the bulk material itself, producing constituent background signals. Compounding this problem, the portion of the gamma ray spectra captured by the detectors that is attributable to the background materials, is not a constant signal because the amount of bulk material inside the measurement zone influences the magnitude of neutron flux impinging on the background materials. Therefore, the errors associated with the unknown magnitude of background signal from the constituents such as H, C, S, N, Ca, Al, Si, and others in the background materials prevent a prior art PGNAA device from accurately reporting only the analysis of the constituents in the bulk material itself. Furthermore, variable amounts of both neutron and gamma ray attenuation caused by variations in the thickness of the bulk material bed also contribute to PGNAA errors because the relative magnitudes of the constituent signals emanating from the bulk material itself are not constant with variable belt loading. In summary the measurement errors are a function of a multitude of parameters, each in some way caused by and related to variations in tonnage or flow rate. For these reasons, the prior art method of simply subtracting constant values of background from each measured constituent will not achieve measurement accuracy in PGNA analyzer applications operating under variable material flow conditions.
In prior applications of PGNA analyzers, considerable cost and effort has been applied in the industry to achieve near-constant flow rate and mass thickness. Such means have included the use of surge hoppers and constant flow feeders, variable speed conveyor belt drives, and vertical, plug-flow type PGNA analyzers, all so as to deliver a constant mass of bulk material (kg/m) or material cross-section to the measurement region.
In prior art, a PGNA analyzer is conventionally calibrated using a set of two or more unique and well-known mixtures of (1) high-purity base materials the amounts of which are carefully weighed prior to mixing, or (2) well-homogenized mixtures of representative unknowns that have been blended, sampled and analyzed for element or compound analysis by conventional laboratory means. In either case the chemistry of each standard is known quite accurately and allow the set of standards to be utilized as a general calibration of the PGNA apparatus. In prior art each standard in the set contains the same fixed mass and length. The entire mass of the standard fills the analyzing volume, and is either contained in a single structural unit, or bound together as a single unit by cement or epoxy block.
The length of the standards is determined by (1) the length of the analyzing volume or (2) a length which if longer, would yield no additional analysis signal. The mass of standard material in each set is configured to match the average material loading (kg/m) the PGNA analyzer will measure in the application. These single mass standards are designed for insertion into the analyzer for the purpose of calibrating the analyzer over the number and range to elemental compositions expected to pass through the analyzer over the course of its service.
Testing and calibration of analyzers using prior art standards can only be done at one fixed flow rate (TPH) or mass loading (kg/m). This is a serious limitation to properly calibrate an analyzer over a range of flow rates, when such PGNA analyzing apparatus is inherently sensitive to amount of material it is measuring due to unknown contributions of background terms.
This invention is directed towards PGNAA devices analyzing variable loads. The invention describes a method of quantifying the analytical measurement error resulting from material mass loadings different from the loading at which the analyzer was calibrated, and a method of compensation for this error, also referred to as xe2x80x9canalysis biasxe2x80x9d, in order to produce accurate analysis results under all load conditions.
The inventive process described herein provides for measurement compensation of on-line Prompt Gamma Neutron Activation Analysis (PGNAA) equipment under conditions of variable flow rate or material mass loading. The process includes utilization of (1) one or more sets of calibration materials in the form of standard geometric units that can be arranged to represent the variations in flow rate and mass loading as well as material geometric profile, (2) a method to quantify the measurement errors of each constituent analyzed over a range of material loadings, and (3) a method to predict by calculation the amount of expected error for each constituent at any given flow rate or loading, and (4) a method to calculated and remove the expected errors for each constituent measured by utilizing the real-time flow rate or tons per hour signal, together with the mathematical functions and parameters derived in points (1) to (3). This process greatly improves PGNA analyzer measurement accuracy and is particularly well suited for industrial applications. The process can be utilized on new or existing PGNAA equipment.